The Soviet-Finnish war was fleeting. Its beginning was in November 1939. After 3.5 months it was over.
The Soviet-Finnish war, the causes of which are still doubtful, was provoked by the Mayil incident, when Soviet border guards were fired from Finnish territory in the village of Mainil. Soviet authorities claimed that this event took place. The Finnish side denied participating in the shelling. Two days later, the Soviet Union unilaterally canceled the non-aggression pact with Finland and unleashed hostilities.
The real causes of the war lay somewhat deeper than the shelling on the border. Firstly, the Soviet-Finnish War was a continuation of the Finnish attacks on Russian territory from 1918 to 1922. As a result of these skirmishes, the parties came to peace and signed an agreement on the inviolability of the border. Finland received the Pecheneg region and part of the Middle and Fishing Islands.
Since then, relations between countries have remained strained, despite the nonaggression pact. Finland was afraid that the USSR would try to regain its lands, while in the USSR it was assumed that the opponent would let in the forces of another unfriendly country that would carry out the attack.
In Finland, during this period, the activities of the Communist Party were banned, and they were actively preparing for war, and the Soviet Union was taking this country to its zone of influence according to the secret protocols of the Molotov-Ribbentrop Pact.
In the same period, the USSR seeks that part of the Karelian Isthmus be exchanged for Karelian territory. But Finland does not agree with the terms and conditions. Negotiations practically did not advance, dropping to mutual insults and reproaches. When they reached an impasse, Finland announced a general mobilization. Two weeks later, the Baltic Fleet and the Leningrad Military District began preparations for hostilities.
The Soviet press launched active anti-Finnish propaganda, which instantly found a corresponding response in the enemy country. The Soviet-Finnish war has finally matured. She has less than a month left.
Many believe that the shelling at the border was an imitation. It is likely that the Soviet-Finnish war, the causes and reasons of which boiled down to this shelling, began with unfounded allegations or provocations. No documentary evidence found. The Finnish side insisted on a joint investigation, but the Soviet authorities sharply rejected this proposal.
Formal ties with the Finnish government were interrupted as soon as the war broke out.
The attacks were planned to be deployed in two directions. Having made a successful breakthrough, the Soviet troops could take advantage of their undeniable power superiority. The command of the Army hoped to carry out the operation from two weeks to a month. The Soviet-Finnish war was not supposed to drag out.
Subsequently, it turned out that the leadership of the Soviet Army had very poor ideas about the enemy. Successfully begun hostilities slowed during the breakthrough of the Finnish defense. Not enough combat power. By the end of December, it became clear that a further offensive under this plan was hopeless.
After significant changes, both Armies were again ready for battle.
The offensive of the Soviet troops continued on the Karelian Isthmus. The Finnish army successfully repulsed them and even attempted counterattacks. But unsuccessfully.
In February, the retreat of Finnish troops began. On the Karelian Isthmus, the Red Army overcame the second defense line. On February 13, Soviet soldiers entered Vyborg.
After that, the Finnish authorities put forward a request to the USSR for negotiations. March 12 was marked by the world according to which the Karelian Isthmus, Vyborg, Sortalava, islands of the Gulf of Finland, the territory with the city of Kuolajärvi and some other territories passed into the possession of the Soviet Union. Finland returned the territory of Petsamo. Also, the USSR was leased territory on the Hanko Peninsula.
At the same time, the confidence of Western countries in the USSR was finally lost. The reason was the Soviet-Finnish war. 1941 began in an extremely difficult situation.